The present invention relates to a method for producing a reflective optical element for EUV lithography, which element has a maximum reflectivity at an operating wavelength in the range of 5 nm to 20 nm.
In EUV lithography apparatuses, reflective optical elements for the extreme ultraviolet (EUV) wavelength range (e.g. wavelengths between approximately 5 nm and 20 nm) such as, for instance, photomasks or mirrors on the basis of multilayer systems are used for the lithography of semiconductor components. Since EUV lithography apparatuses generally have a plurality of reflective optical elements, the latter must have as high a reflectivity as possible in order to ensure a sufficiently high total reflectivity. The reflectivity and the lifetime of the reflective optical elements can be reduced by contamination of the optically used reflective surface of the reflective optical elements that arises on account of the short-wave irradiation together with residual gases in the operating atmosphere. Since a plurality of reflective optical elements are usually arranged one behind another in an EUV lithography apparatus, even relatively small contaminations on each individual reflective optical element affect the total reflectivity to a relatively great extent.
Contamination can occur on account of moisture residues, for example. In this case, water molecules are dissociated by the EUV radiation and the resulting oxygen radicals oxidize the optically active surfaces of the reflective optical elements. A further source of contamination is polymers which can originate from the vacuum pumps used in EUV lithography apparatuses, for example, or from residues of photoresists used on the semiconductor substrates to be patterned and which lead to carbon contaminations on the reflective optical elements under the influence of the operating radiation. While oxidative contaminations are generally irreversible, carbon contaminations, in particular, can be removed, inter alia, by treatment with reactive hydrogen, by virtue of the reactive hydrogen reacting with the carbon-containing residues to form volatile compounds. Reactive hydrogen can be hydrogen radicals or else ionized hydrogen atoms or molecules.